1. Field of the Invention
The present invention relates generally to expansion turbines and, more particularly, to an expansion turbine for producing refrigeration or for use in cryogenic rectification of feed air to produce nitrogen and other gases.
2. Background of the Invention
Industrial gases such as nitrogen and oxygen are produced commercially in large quantities by the cryogenic rectification of feed air. Refrigeration to drive the cryogenic rectification is provided by the turboexpansion of a compressed process stream which is generally either a compressed feed air stream or a high pressure stream taken from the rectification column. The turboexpander of an air separation plant is a costly piece of equipment to operate and maintain and it would be desirable to reduce such costs.
Indeed, the initial cost of turboexpanders used in nitrogen producing facilities represents a sizeable portion of the capital cost of the plant itself. For a small size plant, the cost of the turbine is in the order of 10% of the total plant cost. Reducing the initial cost of the turboexpanders for these applications, then, is desirable for improving the overall plant cost effectiveness. On the other hand, the relative importance of turboexpander performance, or efficiency, cannot be overlooked. Hence, it becomes a classic problem of performance versus cost. Ideally, one would like to have a high performance machine at low cost or, more realistically, a tradeoff between cost and performance. This invention solves the dilemma by offering a low cost machine with reasonable performance, that is, with a machine efficiency up to mid-eighties percent.
Production of nitrogen or oxygen gas through separation of air by cryogenic distillation require use of cryogenic turboexpanders, the design of which is very similar, functionally, to that of a turbocharger. For example, both applications require a turbine stage and a compressor stage connected by a rotatable shaft, mounted in a bearing housing. However, turbochargers, unlike cryogenic turboexpanders, are typically operated at elevated temperatures. This is natural, because the turbocharger was developed to use the exhaust gases discharged from an internal combustion engine as a propellant gas to rotate the turbine wheel, mounted at one end of a shaft. A compressor wheel is mounted at the other end of the shaft, and is turned by the turbine wheel to compress air, which is then communicated to the engine, thereby supplying charge air to the engine for increasing engine performance. The initial cost of a typical turbocharger used in an internal combustion engine is relatively low, because of the advantages of series production. The initial cost of a cryogenic turboexpander, on the other hand, is generally one or two orders of magnitude higher.
There have not previously been significant attempts to solve the cost problem technologically, except through market pressures of supply and demand. Established existing vendors of cryogenic turboexpanders were the suppliers of these machines. More specifically, one could obtain from vendors a lower efficiency machine at a modest cost, or have a high efficiency machine built at high cost. This invention offers an attractive alternative of a low cost machine with modest-to-high performance.
As stated above, the initial cost of the turbine remains prohibitively high. For a small nitrogen product plant useful for producing approximately 25 tons per day for example, the turbine initial cost may be more than 10% of the total plant first cost. The prior art solution to the problem has been technologically deficient, because it offers either high performance at high cost or low performance at modest cost. A machine of modest performance and low cost has not been available.
The use of mass produced subcomponents for constructing an efficient and low cost turboexpander for cryogenic air separation plants has been considered. This approach led to the use of a modified diesel engine turbocharger as a cost effective cryogenic turboexpander. A standard diesel engine turbocharger comprises a turbine stage, rotor, bearings, housing, and a compressor stage. For cryogenic air separation applications, at the minimum, the turbine stage of the turbocharger must be modified to render it suitable for low temperature (cryogenic) service. A turbocharger is customarily designed to operate with hot (above 1,000.degree. F.) exhaust gases of an internal combustion engine. Its application as a cryogenic turboexpander, operating at very low temperatures (below -200.degree. F.), is not only unobvious but even thought impossible, because of the materials of construction, sealing and other constraints.
A primary advantage of the invention resides in its low initial cost which is an order of magnitude less expensive than the current commercially available state-of-the-art turboexpanders. The low cost is possible because of the advantages of series production of turbochargers. An existing turbocharger, however, cannot be used, as is, without embodying the modifications disclosed herein to render it suitable for the cryogenic application. Another advantage is simplicity of design and, associated with it, inherent reliability. Nor do these modifications compromise the performance of the turbine. To the contrary, while the efficiency of the turboexpander of the invention does not exceed existing state-of-the-art turboexpander machinery, the achievable isentropic efficiency in the mid-eighties percent is an excellent combination of low initial cost and relatively good performance resulting in an overall cost advantage over existing state-of-the-art technology. This advantage has merit in many diverse cryogenic plants using a variety air separation cycles.
A primary object of this invention is to provide a low cost turboexpander, which may be effectively employed in a cryogenic production cycle. It is another object of this invention to provide a cryogenic production cycle which can effectively employ such a low initial cost turboexpander of reasonable efficiency.
As indicated, the cryogenic turboexpander of the invention is a rugged machine, with both low initial and maintenance cost and high reliability. Its operating efficiency with a modified turbine wheel is reasonably acceptable for the nitrogen producing plants with waste expansion or air expansion. Some applications, such as oxygen producing air separation plants, or even smaller nitrogen producing plants, may require the use of a state-of-the-art design turbine wheel, which is attached to the rotor instead of the trimmed expander wheel of the original turbocharger, the use of which may be prohibitive because of intolerably low turboexpander efficiency. Even in this situation, however one can obtain the benefits of low machine and maintenance costs.
The low cost turboexpander of this invention can be utilized in plants and cycles producing oxygen and/or nitrogen through separation of air by cryogenic distillation. The machine is especially suited for nitrogen producing plants through separation of air by cryogenic distillation with either a waste expansion or an air expansion cycle. It can be also used in other cryogenic processes, such as hydrogen, natural gas, or similar chemical processes requiring an expansion engine of low first cost and reasonable performance.
Other and further features, advantages, and benefits of the invention will become apparent in the following description taken in conjunction with the following drawings. It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings which are incorporated in and constitute a part of this invention, illustrate one of the embodiments of the invention, and, together with the description, serve to explain the principles of the invention in general terms. Like numerals refer to like parts throughout the disclosure.